STREAM FLOW

Stream flow or discharge, Q, is the volume of water passing a point in a given time. At high flows, large volumes of water are transported through the stream in a given time. Stream flow is measured at a location along a stream from the water velocity and the cross-sectional flow area.

A simple example can illustrate the calculation of stream flow or discharge, Q.

Below is a stream with a rectangular cross-sectional area and a uniform velocity, v.

In this simple stream channel, we would measure stream depth and width. The rectangular cross-sectional area is calculated by multiplying the depth by the width.

Area = A = 4 ft * 5 ft = 20 ft

In this simple stream channel, we can measure the water velocity anywhere as it is the same or uniform at all locations. Water velocity is measured with a stream flow meter. A flow meter has a small propeller-like device which rotates relative to the water velocity. The higher the velocity, the more rotations in a given time. In this example, the velocity has been measured as 1 ft/sec.

Discharge = Q = v * A

Q = 1 ft/sec * 20 ft

Q = 20 cubic ft per sec

Q = 20 cfs

Stream flow or discharge is generally presented in units of cubic feet per second (cfs) in the United States and England.

1 cubic foot = 7.5 gallons

1 cubic foot = 28 liters

In real life, streams do not have rectangular cross-sections. Variable depths are encountered along the stream channel bottom. The cross-sectional area of the stream can be represented by a series of rectangles.

In real life, water velocities within a stream are not uniform. Frictional forces decrease the water velocities near the stream bottom. In stream curves, high water velocities are found on the outside of the curve and low water velocities are found on the inside of the curve.

 

The average velocity with depth in a section of a stream has been found to occur at a depth which is 6/10 ths of the total depth. The flow meter is placed at that depth in each rectangular subsection for measurements. The width and depth of each rectangular subsection is measured. The total stream flow is calculated by summing up flows in each subsection.

Total Q = Flow through Rectangle 1 + Flow through Rectangle 2 + ...+ Flow through Last Rectangle